Microphone.



E. WEINTRAUB & L. B. MILLER.

MICROPHONE.

APPLICATION' FILED 1m31, 1914.

1,156,509-, Patented oet. 1915.

Inventms:

I/VItnsses: Ezehel Weintraub, y. M Levi Exmiller,

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UMTED srArEs 'PA f nzneninr. WEINTRAUB Ann'nnvr ia.` Minnen, oFlLYNN, MASSACHUSETTS, Assronons To GENERAL ELnc'rnIo ooMrANY, e oonPonA'rIoN oF NEW YORK.

` mcnonnonn.

To all whom t may concer/n.:

Be it known that we, EzEoHmL Warn- 'rRAUB andLnvI B. MILLER, citizens of the United States, residing at Lynn, 1n the county of Essex, State of Massachusetts,

. have invented certain new and useful Improvements in Microphones, of which the following is a specification.

The -carbon microphone' such a s commonly used as a telephone transmitter increasesin electrical'rcsistance with contlnued use and l its resistance eventfually'becomes so high asf to make the microphone practically inoperative' 'with .a given` working line voltage. This is especially true of those types oftelephone apparatus in which the transmitter 1s held in hand by the user; and the microphone is assuming diferentpositions wherebythef powder is roughly shaken up.

v Ve have discovered that when carbon having a hardness equal to or greater than 6 on the 'Mohs mineral scale is employed in a microphone that the resistance increases at a very much slower rate than with the softer carbon now used, so that the useful life of the` microphone is lengthened: Other desirable properties of a telephone transmitter, such as clearness of enunciation, are also improved. l

The accompanying drawings-illustrate 1n Figure 1 a common form of telephone transmitter, and in Fig. 2 show in a more dia'- grammatic form a simple microphone which may be used for telephones or othezwpurposes.

The hard carbon used in our improved microphone is preferably prepared from synthetic resins commonly known as condensation products, particularly the phenol= aldehydev resins, some of which' 'are well,- known by trade names, such as bakelite and .specification of Letters Patent.

" rblack, or the like,

The

.ing

Patented oet. 12, 1915.

Application meaianuary e1, 1914. serial No. 815,682.

rises from about A70" C. to'ab'out 290 C. The heating may be carried on in an oven `heated first by steam and then by gas combustion. This' stage ofthe heating produces complete polymerization and hardening of the condensation products and drives out gases. If it is desired to produce disks of thishard carbon such as indicated at 1 and- 2 in the accompanying drawing, the'resin should -be shapedV .into disk form before heating, as shapin subsequent to hardening is diflicult, butA i desired even the hardenedresin may be machined into desired form. After the preliminary bake the .hardened resin is carboniz'ed. p The resin is .preferably packed in a carbon or graphite container in a protective material, such, for example, as

peat. The container, packed in coke, lampb is'slowly heated .by gas combustion or electrically to a temperature rising to about 700 C. i n about one week.v The carbon is given aiinal firing at-a temperature` rising from 800 to 1100 C. in av few hours.

bon powder .Fig-

lzing sifting through screens of suitable mesh. carbon disks are preferably plated 'on one side with copper, or other suitable metal. Ina s1milar manner, buttons of hard carbon,

l, `the carbon produced by this carbonlsuch as indicated at 4, may be used in'con' Junction lwlth a carbon or metal plate 5.

The car'bon is very bright and shiny looksoda-lime glassl and has a conchcidal fracture. A large proportion of the carbon prepared asdescribed above is hard enough' to scratch quartz which on the Mohs scale has ahardness of about 7. Its conductivity may be varied by regulating the firing 'temperature, a higher firing temperature resulting For thepurpose of making caror granules such as shown at 3,

treatment is pulverized and sized by` and is hard enough to scratch ordinaryv in carbon of a greater'conduetivity. vThe granules yof lthis veryhardk carbon can be used in the ordinary microphone transmitter instead of the softer carbon granules. .A standard form oftransmi'tter such Ias shown in Fig. 1` may be used, comprising `a frame 6 supportinga capsule? containing the car- ,bon powderl Thel sound waves directed against'a diaphragm '8 by a mouth-piece 9,

produce a varying' pressure upon the caron powder which is included inthe electrical circuit.

In our opinion the increased lifeV of the microphone is to be ascribed vlargely tojthe greater resistance to Wear and to .pulverizing a fine dust which apparently lis responsible for the increase 4of resistance.v v

To illustrate the improvementobtained in microphones by use of the hard formof carbon, the following results by actual -tests may be described. .The ordinary standard powder was used on -a l8-volt circuit in onel seriesof tests and carbon derived from the phenol-formaldehyde resin known as bakelite in another series. VTo mak-e' the tests severe the transmitter was shaken violently for a half hour by hitting it about once a second in different directions against a solid Wall. 'After half an hour of such treatment the resistance of the standard povvder Went. up from 15() ohms to 450 ohm/s, thus increasingin the ratio 3:1.. The resistance of the microphone With the improved carbon increased from 100 ohms to 140 ohms, or only 40%. This would mean that the useful Ylife ofthe microphone is increased Iive times by the employment of glass-hard carbon. Our new microphone also transmits speech with improved clearness or sharpness of enuncia'fi Jn.

What We claim as new and desire to secure by Letters Patent of the United States, is :-M

1. Av microphone having a 'member con-,-ff sisting' of conductive carbon'made by carbonizing a resinous condensation product, and having hardness at least about 6 on the' .Melis scale.

-ness of at least about 6 on the Mohs scale, a

shiny surface and a conchoidal fracture.

4. A microphone containing vconductive granules of a carbonized resinous phenolic condensation product.

5. Conductive carbon granules, suitable for use in a microphone, having a hardness of at least about 6 on the Mohs scale, a shiny surface and a conchoidal fracture.

yIn ivitness whereof, We have hereunto set ou'r hands this 28th day'of January, 1914.

EZEGHIEL WEINTRAUB-` LEVI B. MILLER.

Witnesses:

JOHN A. McMANUs, Jr., IIENRY A. ANDERSEN.

Copies offthis patent may be obtained for ve cents each, by addressing the Commissioner ofatents..

' Washington, D. C. 

